Generally, a metal gasket is provided with a bead that is compressed and elastically deformed, by a fastening force applied to a tightening bolt for fastening a cylinder head to a cylinder block, to form a seal line on deck surfaces of the cylinder head and the cylinder block which brings a seal therebetween. Conventionally, a metal gasket of this type of metal gasket has been known that has, in order to prevent leakage of high pressure combustion gas in a cylinder bore into an adjacent cylinder or a cooling water hole which might lead to a malfunction such as a decrease in engine output and overheating or a decrease in efficiency of exhaust gas purification due to incomplete combustion gas, a gasket substrate 50 which is composed of a steel plate (e.g. SUS301-H 0.2t) coated with rubber having the thickness of about 0.025 mm on one side or both sides. As shown in FIGS. 6 and 7, the gasket substrate 50 is provided with full beads 52 each having an angle section and arranged around a respective cylinder hole 54, half beads 58 each surrounding a respective oil hole to prevent leakage of fluid such as cooling water and lubricating oil, and a half bead 62 for sealing cooling water holes 60 surrounding all of the full beads 52 around the cylinder holes 54 without enclosing the half beads 58 for oil holes. FIG. 8 shows a metal gasket having a three layered structure formed with three pieces of the above-mentioned gasket substrates 50 stacked up on top of one another.
The reason why the half and full beads are provided on the gasket substrates in accordance with the purpose of sealing is that the restoring forces against the fastening forces from the tightening bolts are different between the half and full beads. That is, the pressure of high-pressure combustion gas in the cylinder reaches 60 to 100 kg/cm2, so that the full bead with relatively larger restoring force which yields large fastening pressure is provided around the cylinder hole to seal the high pressure combustion gas. On the other hand, pressure of fluid such as the cooling water and lubrication oil is about 3 to 6 kg/cm2, so that the fluid can be sufficiently sealed by the half bead having smaller restoring force than that of the full bead.
However, since the outputs of engines have been increased to involve higher pressure of the combustion gas in recent years, better sealing performance of the gasket is thus demanded. Most of metal gaskets having simple stack-up configurations as shown in FIGS. 6 to 8 cannot seal the combustion gas that has been highly pressurized. In particular, even when a cylinder head and a cylinder block of an internal combustion engine with high output are fastened by tightening bolts, a head-lift phenomenon that the cylinder head is raised by the pressure of the combustion gas becomes more noticeable. The head lift phenomenon causes a decrease on fastening pressure of the gasket interposed between the cylinder head and the cylinder block to deteriorate the sealing performance of the gasket against the combustion gas in the cylinder.
As a metal gasket to solve the above described problem concerning the insufficient pressure of the gasket associated with the high engine power output, it has been proposed that, as shown in FIG. 9, a sub-plate 64 that has a thickness difference (step) formed by a thick plate portion T1 and a thin plate portion T2 is interposed between the above-mentioned gasket substrates so that the full beads 52 of the gasket substrates 50 are entirely situated on the thick plate T1, and the fastening pressure of each full beads 52 around the each cylinder hole 54 of the gasket substrate 50 is raised to improve sealing performance for the combustion gases in the cylinders.
For the purpose of reference, stacked-up type metal gaskets as shown in FIGS. 6 to 8 and a metal gasket having a sub-plate interposed between two gasket substrates facing oppositely to each other as shown in FIG. 9 (referred to below simply as the “step-type metal gasket”) were made by way of experiment, and the fastening pressure around the cylinder holes of the respective metal gaskets were measured. The results are shown in FIG. 10. Reference 1 is a metal gasket having a two-layer structure (see FIGS. 6 and 7). Reference 2 is a metal gasket having a three-layer structure (see FIG. 8). References 3 to 6 are metal gaskets having a step-type structure (see FIG. 9). Here, the thickness difference in the plate (the thickness difference between the thick plate portion T1 and the thin plate portion T2, also referred to below simply as the “step difference”) is 0.03 mm in Reference 3, 0.05 mm in Reference 4, 0.08 mm in Reference 5, 0.10 mm in Reference 6. It is shown in FIG. 10 that the metal gaskets of References 1 and 2 that do not have a sub plate do not reach required fastening pressure A (kg/mm) even when an axial force of the clamping bolts has been increased, and, therefore, a sufficient sealing performance for the high-pressure combustion gas cannot be obtained. It is noted that the test was conducted through measuring the fastening pressure when the head lift phenomenon occurred. In particular, the metal gaskets were installed in the engine; clamping bolts are screwed with a predetermined axial force; subsequently, N2 gas was charged into the cylinder at a pressure equivalent to the maximum pressure (kg/cm2) of the combustion gas in the engine to force the head lift to occur; the fastening pressure around the cylinder holes was measured after the occurrence of the head lift; and the measured fastening pressure was assumed as the fastening pressure supported by the full beads.
Concerning the step structure of the sub-plate 64, for example, it has been conventionally known that, as shown in FIG. 11 (a), the step structure of the sub-plate 64 to be interposed between two gasket substrates 50 each consisting of a metal plate is formed such that thin metal plates 64a, 64b having mutually difference plate thicknesses are selected to have a desired thickness difference; the metal plate 64a is positioned to constitute a cylinder hole peripheral portion (thick plate portion T1) overlapping with each full bead 52 surrounding the respective cylinder hole 54 of each gasket substrate 50; the metal plate 64b is positioned to constitute an outer portion (thin plate portion T2) outside of the cylinder hole peripheral portion; and the thin metal plates 64a, 64b are joined each other with laser welding (see Patent Literature 1). It is noted that the reference numeral Yw in the drawing illustrates a laser welded portion.
Furthermore, for example, it has also been known that, as shown in FIG. 11 (b), the step structure is formed such that a metal sub-plate 64 consists of a thin metal plate with a uniform thickness to be interposed between two gasket substrates 50 each consisting of a metal plate; a shim plate 66 also consisting of a metal plate is stacked up on the cylinder hole peripheral portion overlapping with each full bead 52 surrounding the respective cylinder hole 54 of each gasket substrate 50 so as to form a required step; and the sub-plate 64 and the shim plate 66 are joined each other with laser welding.
Yet further, for example, it has also been know that, as shown in FIG. 11 (c), the step structure is formed such that a metal sub-plate 64 with a uniform thickness to be interposed between two gasket substrates 50 each consisting of a metal plate is thinned by etching at the above-mentioned outer portion to form the required thickness difference between the cylinder hole peripheral portion (thick plate portion T1) and the outer portion (thin plate portion T2).    Patent Literature 1: Japanese Patent Application Publication No. 1995-243531.